"eventually led to the idea of" is a phrase of a reviewer. Tom Holland is very emphatic that the Canossa episode was a significant crossroad in European history. Islam may have been similarly dynamic at that time, but the dispute between the Henries and the Popes was a special development that defined European norms and enabled a more dynamic trajectory. Sunni complexities remained chronological facts somehow.

Our general discussion is not just about before the Mongol invasion (especially if you still pivot on the real separation). It is surely an open question how Christianity would have coped with the Mongols if they would have seen Europe worthwhile to conquer. But the Mongol impact on the Middle East soil and ideology was pretty arresting.